Synthetic aperture radars (SARs) are widely used in the activities of remote analysis of the physical characteristics of terrestrial and maritime zones on the basis of their radar reflectivity. These activities are called “Radar remote sensing”. One of the activities in this field is the detection of hydrocarbon oil slicks on the sea by difference of radar reflectivity level between the polluted zones and the unpolluted zones.
Studies have been carried out to evaluate the disturbance introduced by the presence of rain in respect of this analysis. Such is the case for example for the study “Rain-induced modification of SAR performance” by P. Ferrazzoli and G. Schiavon—Adv Space Res vol 7 No. 11 1987. In this study, the defects related to the presence of rain are presented but the analysis performed does not deal with the fact that the rain echoes may have a Doppler spectrum that is shifted with respect to the spectrum of the ground echoes of the observed zone. Moreover, this study does not propose any solution for reducing the disturbance.
Other studies deal with the impact of the presence of moving echoes in the zone observed by an SAR radar (see notably “Principles and applications of imaging radars”—Manual of Remote Sensing—Third Edition—Volume 2) and propose solutions to enable detection of moving echoes on the basis of SAR processing. However, these techniques, which are akin to the techniques for detecting moving targets (or MTI for Moving Target Indicator), are effective only for the detection of targets having a radial velocity sufficient for their Doppler frequency to exit the Doppler spectrum of the fixed echoes observed by the SAR technique. These techniques are not applicable in the case where the domain of variation of the relative radial velocities of the disturbing elements such as rain drops is not clear of the domain of the relative radial velocities of the elements of the observed scene which are at the same geographical position.
Techniques for improving the texture of images of maritime, rural or forest SAR scenes have also been described notably in the work “Principles and applications of imaging radars” cited previously. These procedures are applied to already constructed SAR images. They rely on the construction of the 2D spectrum of the SAR image and on the elimination of a part of the spectrum which corresponds to speckle, so as to retain only the elements having a particular texture like the pattern of waves. These procedures are applicable only when the expected image has a particular texture. Such is not the case, for example, when observing a clear calm sea, at low grazing angle under horizontal polarization.
Finally, in situations where the phenomenon of interest does not lead to images having a particular texture, segmentation procedures can be used. These procedures are based on the identification of probability density laws which differ on the background of the image and on the zone sought.
This segmentation can also be complemented with so-called “mathematical morphology” operations making it possible to smooth the image resulting from the segmentation, starting from the assumption that the zones sought have an extent of several pixels of the image.
A drawback of these solutions is that they are applied once the SAR image has been created and the spurious phenomenon has contributed to the increasing of all the levels of the pixels corresponding to zones of low reflectivity that it is sought to identify on the map and therefore to reducing the contrast between the zones of low reflectivity and the zones of lower reflectivity.
An aim of the invention is notably to correct all or some of the aforementioned drawbacks by proposing an SAR imaging processing method making it possible to attenuate the effects of spurious echoes.